I recently bought Doshu's new tape-the last one which shows oyo waza and some kihon waza. He demonstrates yonkyo on the tape.
He seems to use the mechanical leverage with the application of pressure points in two ways: one on the forearm bone along the radius near where the course of the radial nerve runs and two with his supporting hand he presses his thumb into the wrist.
Personally, I like to use both the mechanical and the pressure point only I add a complex torque to the wrist to accentuate access to the nerve either via a sankyo or mawashi wrist bend. This seems to expose the nerve or bone-whatever hurts by moving some of the tendons out of the way.